I used to play osrs on a macbook pro 2 and that worked very smooth so I'm sure on a M3 it also works very smooth so I wouldn't worry about runescape.
Only thing is that if you were always used to having windows switching to MacOS is just shit, I am a very handy guy with computers but when I moved from a windows computer to the newest macbook pro I truthly regretted it after 1 year of using it and eventually sold my macbook to go back to windows. Two others friends also switched to a macbook pro going from a windows desktop because of study and both eventually sold their macbooks and went to a windows laptop and regretted their purchase. We all three still makes jokes about it lol.
MacOS is good if your a simple computer guy who just check mail watch some youtube do some work other then that its shit.